From the Siemens product document “ISDN im Büro HICOM”, special edition of telecom report and Siemens magazine COM, 1985, ISBN 3-8009-3849-9, in particular pages 26 to 31 and 58 to 66, is known a communications system which provides additional functions apart from switching-related functions. Additional functions of this type are generally called features, with a known feature being what is referred to as a “presence service”. By virtue of the “presence service” feature a user is shown on a terminal with which he is associated, for example a telephone or personal computer, information about the current state of other users or their terminals. This information is often organized in the form of a list—often called a Buddy List in the literature. The users on the list can usually be freely configured, for example all people working on a project or the employees of a department can be summarized in a list of this type.
To be able to achieve a real-time presence service it is necessary for all changes in the status—for example taking up or replacing a telephone receiver in the case of a telephone terminal—of the monitored user to be identified and change messages to be transmitted to the corresponding terminals to update the lists. This can sometimes lead to high loading of the system. If, for example, a system with 50 users per list, 20,000 devices to be monitored, 10 updates per hour and user and a transmission rate of 1 kbyte per update, is taken as a starting point, then this alone leads to a system load of 22.76 Mbit/s for the change messages.